In an electrophotographic process, to keep the toner concentration constant in two-component developer containing toner and carrier is very important for maintaining stable image formation quality.
As a technology for keeping the toner concentration constant, a technology of measuring the number of basic image points of image data used for printing data and controlling toner supply according to the measurement value is known. The conventional technology is to supply an amount of toner to be used or an amount of used toner to a developing unit.
Regarding the conventional toner supply system as described above, a method of correcting an amount of toner supply, if the toner concentration detected by toner concentration detecting means is off the target value, to bring the toner concentration close to the target value according to an amount of difference from the target value is disclosed (e.g., see Japanese Patent Publication No. 2942017, Japanese Patent Publication No. 3072783, or the like).
However, the conventional technology employs a complex method of calculating the amount of toner supply based on the number of pixels of image data used for printing, calculating the amount of correction according to the toner concentration in the developer, and calculating a new amount of toner supply by computation of the amount of toner supply based on the number of pixels and the amount of correction.
A sprinting processing in a high-speed printer (e.g., a so-called four-drum tandem type image forming apparatus), under the condition that calculation of plural times of toner supply in a short period is required, it is difficult to realize the calculation of the amount of toner supply in the above conventional technology at low cost because a high-speed arithmetic processing unit is needed.